Field of the Invention
This invention relates to a method for reducing sheeting during polymerization of alpha olefins and more particularly to a method for reducing sheeting during polymerization of polyethylene.
Summary of the Prior Art
Conventional low density polyethylene has been historically polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. or higher. The molecular structure of high pressure, low density polyethylene (HP-LDPE) is highly complex. The permutations in the arrangement of their simple building blocks are essentially infinite. HP-LDPE's are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of these resins. HP-LDPE's also possess a spectrum of short chain branches, generally 1 to 6 carbon atoms in length. These short chain branches disrupt crystal formation and depress resin density.
More recently, technology has been provided whereby low density polyethylene can be produced by fluidized bed techniques at low pressures and temperatures by copolymerizing ethylene with various alpha-olefins. These low pressure LDPE (LP-LDPE) resins generally possess little, if any, long chain branching and are sometimes referred to as linear LDPE resins. They are short chain branched with branch length and freguency controlled by the type and amount of comonomer used during polymerization.
As is well known to those skilled in the art, low pressure, high or low density polyethylenes can now be conventionally provided by a fluidized bed process utilizing several families of catalysts to produce a full range of low density and high density products. The appropriate selection of catalysts to be utilized depends in part upon the type of end product desired, i.e., high density, low density, extrusion grade, film grade resins and other criteria.
The various types of catalysts which may be used to produce polyethylenes in fluid bed reactors can generally be typed as follows:
Type I. The silyl chromate catalysts disclosed in U.S. Pat. No. 3,324,101 to Baker and Carrick and U.S. Pat. No. 3,324,095 to Carrick, Karapinks and Turbet. The silyl chromate catalysts are characterized by the presence therein of a group of the formula: ##STR1## wherein R is a hydrocarbyl group having from 1 to 14 carbon atoms. The preferred silyl chromate catalysts are the bis(triarylsilyl) chromates and more preferably bis(triphenylsilyl) chromate.
This catalyst is used on a support such as silica, alumina, thoria, zirconia and the like, other supports such as carbon black, micro-crystalline cellulose, the non-sulfonated ion exchange resins and the like may be used.
Type II. The bis(cyclopentadienyl) chromium (II) compounds disclosed in U.S. Pat. No. 3,879,368. These bis(cyclopentadienyl) chromium (II) compounds have the following formula: ##STR2## wherein R' and R" may be the same or different C.sub.1 to C.sub.2, inclusive, hydrocarbon radicals, and n' and n" may be the same or different integers of 0 to 5, inclusive. The R' and R" hydrocarbon radicals may be saturated or unsaturated, and can include aliphatic, alicyclic and aromatic radicals such as methyl, ethyl, propyl, butyl, pentyl, cyclopentyl, cyclohexyl, allyl, phenyl and naphthyl radicals.
These catalysts are used on a support as heretofore described.
Type III. The catalysts as described in U.S. Pat. No. 4,011,382. These catalysts contain chromium and titanium in the form of oxides and, optionally, fluorine and a support. The catalysts contain, based on the combined weight of the support and the chromium, titanium and fluorine, about 0.05 to 3.0, and preferably about 0.2 to 1.0, weight percent of chromium (calculated as Cr), about 1.5 to 9.0, and preferably about 4.0 to 7.0, weight percent of titanium (calculated as Ti), and 0.0 to about 2.5, and preferably about 0.1 to 1.0 weight percent of fluorine (calculated as F).
The chromium compounds which may be used for the Type III catalysts include CrO.sub.3, or any compound of chromium which is oxidizable to CrO.sub.3 under the activation conditions employed. At least a portion of the chromium in the supported, activated catalyst must be in the hexavalent state. Chromium compounds other than CrO.sub.3 which may be used are disclosed in U.S. Pat. No. 2,825,721 and U.S. Pat. No. 3,622,521 and include chromic acetyl acetonate, chromic nitrate, chromic acetate, chromic chloride, chromic sulfate, and ammonium chromate.
The titanium compounds which may be used include all those which are oxidizable to TiO.sub.2 under the activation conditions employed, and include those disclosed in U.S. Pat. No. 3,622,521 and Netherlands Patent Application No. 72-10881.
The fluorine compounds which may be used include HF, or any compound of fluorine which will yield HF under the activation conditions employed. Fluorine compounds other than HF which may be used are disclosed in Netherlands Patent Application No. 72-10881.
The inorganic oxide materials which may be used as a support in the catalyst compositions are porous materials having a high surface area, that is, a surface area in the range of about 50 to 1000 square meters per gram, and an average particle size of about 20 to 200 microns. The inorganic oxides which may be used include silica, alumina, thoria, zirconia and other comparable inorganic oxides, as well as mixtures of such oxides.
Type IV. The catalysts as described in U.S. Pat. No. 4,302,566 in the names of F. J. Karol et al, and entitled, "Preparation of Ethylene Copolymers in Fluid Bed Reactor" and assigned to the same assignee as the present application. These catalysts comprise at least one titanium compound, at least one magnesium compound, at least one electron donor compound, at least one activator compound and at least one inert carrier material.
The titanium compound has the structure EQU Ti (OR).sub.a X.sub.b
wherein R is a C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radical, or COR' where R' is a C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radical; X is Cl, Br, or I; a is 0 or 1; b is 2 to 4 inclusive; and a+b=3 or 4.
The titanium compounds can be used individually or in combination thereof, and would include TiCl.sub.3, TiCl.sub.4, Ti(OCH.sub.3)Cl.sub.3, Ti(OC.sub.6 H.sub.5)Cl.sub.3, Ti(OCOCH.sub.3)Cl.sub.3 and Ti(OCOC.sub.6 H.sub.5)Cl.sub.3.
The magnesium compound has the structure: EQU MgX.sub.2
wherein X is Cl, Br, or I. Such magnesium compounds can be used individually or in combinations thereof and would include MgCl.sub.2, MgBr.sub.2 and MgI.sub.2. Anhydrous MgCl.sub.2 is the preferred magnesium compound.
The titanium compound and the magnesium compound are generally used in a form which will facilitate their dissolution in the electron donor compound.
The electron donor compound is an organic compound which is liquid at 25.degree. C. and in which the titanium compound and the magnesium compound are partially or completely soluble. The electron donor compounds are known as such or as Lewis bases.
The electron donor compounds would include such compounds as alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ethers, cyclic ethers and aliphatic ketones.
The catalyst may be modified with a boron halide compound having the structure: EQU BR.sub.c X'.sub.3-c
wherein R is an aliphatic or aromatic hydrocarbon radical containing from 1 to 14 carbon atoms or OR', wherein R' is also an aliphatic or aromatic hydrocarbon radical containing from 1 to 14 carbon atoms; X' is selected from the group consisting of Cl and Br, or mixtures thereof, and; c is 0 or 1 when R is an aliphatic or aromatic hydrocarbon and 0, 1 or 2 when R is OR'.
The boron halide compounds can be used individually or in combination thereof, and would include BCl.sub.3, BBr.sub.3, B(C.sub.2 H.sub.5)Cl.sub.2, B(OC.sub.2 H.sub.5)Cl.sub.2, B(OC.sub.2 H.sub.5).sub.2 Cl, B(C.sub.6 H.sub.5)Cl.sub.2, B(OC.sub.6 H.sub.5)Cl.sub.2, B(C.sub.6 H.sub.13)Cl.sub.2, B()C.sub.6 H.sub.13)Cl.sub.2, and B(OC.sub.6 H.sub.5).sub.2 Cl. Boron trichloride is the particularly preferred boron compound.
The activator compound has the structure: EQU Al(R").sub.c X'.sub.d H.sub.e
wherein X' is Cl or OR.sub.1 ; R.sub.1 and R" are the same or different and are C.sub.1 to C.sub.14 saturated hydrocarbon radicals, d is 0 to 1.5, e is 1 or 0, and c+d+e=3.
Such activator compounds can be used individually or in combinations thereof.
The carrier materials are solid, particulate materials and would include inorganic materials such as oxides of silicon and aluminum and molecular sieves, and organic materials such as olefin polymers, e.g., polyethylene.
Type V. Vanadium based catalysts. These type catalysts generally include vanadium as the active ingredient, one such type catalyst generally comprises a supported precursor, a cocatalyst and a promoter. The supported precursor consists essentially of a vanadium compound and modifier impregnated on a solid, inert carrier. The vanadium compound in the precursor is the reaction product of a vanadium trihalide and an electron donor. The halogen in the vanadium trihalide is chlorine, bromine or iodine, or mixtures thereof. A particularly preferred vanadium trihalide is vanadium trichloride, VCl.sub.3.
The electron donor is a liquid, organic Lewis base in which the vanadium trihalide is soluble. The electron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic esters, aliphatic ketones, aliphatic amines, aliphatic alcohols, alkyl and cycloalkyl ethers, and mixtures thereof. Preferred electron donors are alkyl and cycloalkyl ethers, including particularly tetrahydrofuran. Between about 1 to about 20, preferably between about 1 to about 10, and most preferably about 3 moles of the electron donor are complexed with each mole of vanadium used.
The modifier used in the precursor has the formula: EQU MX.sub.a
wherein:
M is either boron or AlR.sub.(3-a) and wherein each R is independently alkyl, provided that the total number of aliphatic carbon atoms in any one R group may not exceed 14; PA1 X is chlorine, bromine or iodine; and PA1 a is 0, 1 or 2, with the provision that when M is boron a is 3. PA1 R' is hydrogen or unsubstituted or halosubstituted lower, i.e., up to about C.sub.6 containing, alkyl; PA1 X' is halogen; and PA1 b is 0, 1 or 2.
Preferred modifiers include C.sub.1 to C.sub.6 alkyl aluminum mono and di- chlorides and boron trichloride. A particularly preferred modifier is diethyl aluminum chloride. About 0.1 to about 10, and preferably about 0.2 to about 2.5, moles of modifier are used per mole of electron donor.
The carrier is a solid, particulate porous material inert to the polymerization. The carrier consists essentially of silica or alumina, i.e., oxides of silicon or aluminum or mixtures thereof. Optionally, the carrier may contain additional materials such as zirconia, thoria or other chemically inert to the polymerization or mixtures thereof.
The carrier is used as a dry powder having an average particle size of between about 10 to about 250, preferably about 20 to about 200, and most preferably about 30 to about 100, microns. The porous carrier has a surface area of greater than or equal to about 3, and preferably greater than or equal to about 50, m.sup.2 /g. A preferred carrier is silica having pore sizes of greater than or equal to about 80, and preferably greater than or equal to about 100, angstroms. The carrier is predried by heating to remove water, preferably at a temperature of greater than or equal to about 600.degree. C.
The amount of carrier used is that which will provide a vanadium content of between about 0.05 to about 0.5 mmoles of vanadium per gram (mmole V/g), and preferably between about 0.2 to about 0.35 mmole V/g, and most preferably about 0.29 mmole V/g.
The carrier is ordinarily free of preparative chemical treatment by reaction with an alkylaluminum compound prior to the formation of the supported precursor. Such treatment results in the formation of aluminum alkoxides chemically bonded to the carrier molecules. It has been discovered that the use of such a treated carrier in the catalyst composition and process is not only nonessential, but instead results in undesirable agglomeration when used in the preparation of high density polyethylene (&gt;0.94 g/cc), resulting in a chunk-like, non-freely flowing product.
The cocatalyst which can be employed for the Type IV and Type V catalysts has the formula: EQU AlR.sub.3
wherein R is as previously defined in the definition of M. Preferred cocatalysts include C.sub.2 to C.sub.8 trialkylaluminum compounds. A particularly preferred cocatalyst is triisobutyl aluminum. Between about 5 to about 500, and preferably between about 10 to about 50, moles of cocatalyst are used per mole of vanadium.
The promoter has the formula: EQU R'.sub.b CX'.sub.(4-b)
wherein:
Between about 0.1 to about 10, and preferably between about 0.2 to about 2, moles of promoter are used per mole of cocatalyst.
The catalyst is produced by first preparing the supported precursor. In one embodiment, the vanadium compound is prepared by dissolving the vanadium trihalide in the electron donor at a temperature between about 20.degree. C. up to the boiling point of the electron donor for a few hours. Preferably, mixing occurs at about 65.degree. C. for about 3 hours or more. The vanadium compound so produced is then impregnated onto the carrier. Impregnation may be effected by adding the carrier as a dry powder or as a slurry in the electron donor or other inert solvent. The liquid is removed by drying at less than about 100.degree. C. for a few hours, preferably between about 45.degree. to about 90.degree. C. for about 3 to 6 hours. The modifier, dissolved in an inert solvent, such as a hydrocarbon, is then mixed with the vanadium impregnated carrier. The liquid is removed by drying at temperatures of less than about 70.degree. C. for a few hours, preferably between about 45.degree. to about 65.degree. C. for about 3 hours.
The cocatalyst and promoter are added to the supported precursor either before and/or during the polymerization reaction. The cocatalyst and promoter are added either together or separately, and either simultaneously or sequentially during polymerization. The cocatalyst and promoter are preferably added separately as solutions in inert solvent, such as isopentane, during polymerization.
In general, the above catalysts are introduced together with the polymerizable materials, into a reactor having an expanded section above a straight sided section. Cycle gas enters the bottom of the reactor and passes upward through a gas distributor plate into a fluidized bed located in the straight sided section of the vessel. The gas distributor plate serves to ensure proper gas distribution and to support the resin bed when gas flow is stopped.
Gas leaving the fluidized bed entrains resin particles. Most of these particles are disengaged as the gas passes through the expanded section where its velocity is reduced.
In order to satisfy certain end use applications for ethylene resins, such as for film, injection molding and roto molding applications, catalyst Types IV and V with alkyl aluminum cocatalysts have been used. However, attempts to produce certain ethylene resins utilizing alkyl aluminum cocatalysts with the Type IV and V catalysts supported on a porous silica substrate in certain fluid bed reactors, have not been entirely satisfactory from a practical commercial standpoint. This is primarily due to the formation of "sheets" in the reactor after a period of operation. The "sheets" can be characterized as constituting a fused polymeric material.
It has been found that a static mechanism is a contributor to the sheeting phenomena whereby catalyst and resin particles adhere to the reactor walls due to static forces. If allowed to reside long enough under a reactive environment, excess temperatures can result in particle fusion. Numerous causes for static charge exist. Among them are generation due to frictional electrification of dissimilar materials, limited static dissipation, introduction to the process of minute quantities of prostatic agents, excessive catalyst activities, etc. Strong correlation exists between sheeting and the presence of excess static charges either negative or positive. This is evidenced by sudden changes in static levels followed closely by deviation in temperatures at the reactor wall. These temperature deviations are either high or low. Low temperatures indicate particle adhesion causing an insulating effect from the bed temperature. High deviations indicate reaction taking place in zones of limited heat transfer. Following this, disruption in fluidization patterns is generally evident, catalyst feed interruption can occur, product discharge system pluggage results, and thin fused agglomerates (sheets) are noticed in the granular product.
The sheets vary widely in size, but are similar in most respects. They are usually about 1/4 to 1/2 inch thick and are from about one to five feet long, with a few specimens even longer. They have a width of about 3 inches to more than 18 inches. The sheets have a core composed of fused polymer which is oriented in the long direction of the sheets and their surfaces are covered with granular resin which has fused to the core. The edges of the sheets can have a hairy appearance from strands of fused polymer.
It is therefore an object of the present invention to provide a method for substantially reducing or eliminating the amount of sheeting which occurs during the low pressure fluidized bed polymerization of alpha-olefins utilizing titanium based compounds or vanadium based compounds as catalyst with alkyl aluminum as cocatalysts.
These and other objects will become readily apparent from the following description taken in conjunction with the accompanying drawing which generally indicates a typical gas phase fluidized bed polymerization process for producing high density and low density polyolefins slightly modified to reflect the present invention.